From a3004fa8b5c4d891132c5b8bc1dafd7964a88484 Mon Sep 17 00:00:00 2001
From: noti0na1 <8036790+noti0na1@users.noreply.github.com>
Date: Wed, 10 Apr 2024 12:58:51 +0000
Subject: [PATCH] Optimize comparing types in mergeRefinedOrApplied

[Cherry-picked 4b711f584e37cd751b813d338a5f65ab09ba9fcb]
---
 compiler/src/dotty/tools/dotc/core/TypeOps.scala | 4 +++-
 tests/pos/i19789.scala                           | 5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i19789.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeOps.scala b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
index 6f323c1c0e4d..1f070d988318 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
@@ -255,7 +255,8 @@ object TypeOps:
           mergeRefinedOrApplied(tp1, tp21) & mergeRefinedOrApplied(tp1, tp22)
         case _ =>
           fail
-      tp1 match {
+      if tp1 eq tp2 then tp1
+      else tp1 match {
         case tp1 @ RefinedType(parent1, name1, rinfo1) =>
           tp2 match {
             case RefinedType(parent2, `name1`, rinfo2) =>
@@ -279,6 +280,7 @@ object TypeOps:
           }
         case AndType(tp11, tp12) =>
           mergeRefinedOrApplied(tp11, tp2) & mergeRefinedOrApplied(tp12, tp2)
+        case tp1: TypeParamRef if tp1 == tp2 => tp1
         case _ => fail
       }
     }
diff --git a/tests/pos/i19789.scala b/tests/pos/i19789.scala
new file mode 100644
index 000000000000..24c3bdb1df8f
--- /dev/null
+++ b/tests/pos/i19789.scala
@@ -0,0 +1,5 @@
+type Kinded[F[_]] = F[Any] | F[Nothing]
+
+def values[F[_]]: Vector[Kinded[F]] = ???
+
+def mapValues[F[_], T](f: Kinded[F] => T): Vector[T] = values[F].map { case x => f(x) }